128 PRACTICAL ORGANIC CHEMISTRY 4. Add to a solution of urea a few drops of hydro chloric acid and a solution of sodium nitrite. Effervescence occurs and nitrogen and carbon dioxide are evolved. CO(NH2)2 + 2HO.NO = 2 No + CO2 = 3^2^' 5. Heat a little urea with soda-lime. Ammonia is evolved. See Appendix, p. 267. PREPARATION 39. Thiocarbamide (Thiourea), Reynolds, Trans. C/iem. Soc., 1869, 22, r ; Volhard, JT. $rakt. Chem., 1874, (2), 9, 10. 50 grms. ammonium thiocyanate« The ammonium thiocyanate is melted in a round flask in n paraffin-bath, and kept at a temperature at which the mass re- mains just liquid (140—145°) for 5—6 hours. The cooled melt is powdered and ground with half its weight of cold water, which dissolves unchanged ammonium thiocyanate, but little of the thiourea. By dissolving* the residue in a little hot water, pure thiourea is obtained, on cooling, in colourless, silky needles. Yield 7—8 grams. CNS.NH4 = CS(NH2)2. Properties—Colourless, rhombic prisms (from dilute aqueous solution), long silky needles (from concentrated solutions) ; m. p. 172°. Very slightly soluble in cold water (i part of thiourea dis- solves in about n parts of water at the ordinary temperature). HN—CO Uric Acid, CO C—NH I |[ >CO HN—C—NH Scheele (1776). Uric acid is a product of the metabolism of trie animal organism. It is usually prepared from guano, which is treated first with dilute hydrochloric acid to remove phosphate of cal- cium. The uric acid is then dissolved out with hot caustic soda and the clear alkaline solution precipitated with acid.